In the milling process several mills, each being comprised of three compression rollers, (although there are some that use only two and others that use as many as five), compress the cane between the rollers causing the cane juice to be extracted. Usually water or diluted juice from a following mill is added ahead of a given mill to aid in the extraction process. The mills are arranged sequentially in what is called a milling tandem resulting in a series process in which the cane, previously chopped up by knives or shredders, enters the first of these mills. In the first mill, the chopped or shredded cane is compressed, partially extracting the sugar-containing juices and then discharged to a conveying means to be similarly treated by a second mill, then a third, then a fourth, and so forth. The number of mills in a tandem vary, but there are usually four to seven in a tandem.
There are two principal techniques as applied to cane milling with roller mills. In one the material discharged by a preceding mill is conveyed to a hopper located ahead of the following mill and from which this mill feeds. This technique requires a high degree of cane disintegration by shredders ahead of the milling process. It also requires that a large amount of water or diluted juice be added to the cane between mills to cause it to be in a semi-fluid form so that it can flow properly from the hopper into the mill.
The other principal milling technique for roller milling tandems requires less cane preparation or shredding ahead of the milling process as well as less liquid application between mills, but for proper milling performance this second milling technique requires that the material be delivered to the mill entrance in the form of a feed blanket of a width closely equal to the cylindrical length of the mill rollers.
The capacity of a milling tandem is often limited by only some of its mills, such as would be the case of milling tandems using some older and some newer mills, the older units imposing their limitation on the new units due to the series nature of the milling operation. Limitation in capacity is imposed on a milling tandem also as a result of the high moisture in the bagasse being discharged by the last mill of a tandem grinding at relatively large capacity since this moisture content must be within certain levels for the bagasse to burn properly as a fuel in the boiler furnaces that generate the power and steam required to process the cane into sugar.
It should be borne in mind that in the conventional milling process referred to, considerable emphasis is placed in the formation of a uniform feed blanket at the beginning of the milling process and to its maintenance throughout this milling process.
In the past, due to the series nature of the milling process together with the conveying means for the material presently being used between mill, when the flow of material thru any given mill is interrupted, it results in a total milling tandem stoppage, or as in the case of a factory with a single tandem, a total factory shut down, a fairly common occurrence which is often a main cause of lost production for sugar cane factories.
With the conveying devices between mills presently used, which dead end at the entrance of the following mill, thruput must be kept below such level that would cause a momentary over feeding of any of the mills in the tandem since, if this occurs, the mills thus overfed will choke, or refuse all feed, causing shut down of variable duration and therefore lost production.
In the following description the term bagasse is used when referring to the fibrous residue of the cane after having been processed by a first set of mill rolls and any mill thereafter.